The invention relates to a method and apparatus for making bituminous mixtures. More specifically, the invention relates to compounding a bituminous mixture of substantially moist aggregates and a hot bituminous binder, in which the aggregates and the bituminous binder are brought together at a predetermined mixing temperature in a mixing tank provided with mixing tools and are mixed to form a bituminous construction material, in particular for bituminous road construction. The aggregates used may range in their grain sizes from a powdery consistency to granular solids of mineral origin. A possible bituminous binder may be a thermoplastic binder based on petroleum or coal.
Processes for the compounding of bituminous mixtures have been disclosed in practice, and may essentially be divided into the following two groups. First, there are processes in which the solids are dried and heated before the bituminous binder, also heated and thereby liquified, is added, whereby the temperature of the solids is within the range of 130.degree.-160.degree. C.
As an example of these processes, there may be mentioned the known methods for the production of hot asphalt for road construction using bitumen as the binder. In this context, reference is made to "Strassenbau mit Shellbitumen" ("Road Construction Using Shell Bitumen"), 3rd Edition, 1954, by Deutsche Shell Aktiengesellschaft and published by Deutsche Shell Aktiengesellschaft, Hamburg, pages 42 and 43. According to the compounding schedule described in this publication the sands and crushed rocks are metered together into the drum dryer and, after drying and heating, are introduced at intervals, with the filler, into a weighing scale which empties into the mixer. The bituminous binder is introduced into the mixer, either at the same time as the other material or after the other material has been added.
This process has the disadvantage that the solids, which during drying must be heated to temperatures beyond the processing temperature of the particular binder, are exposed to abrasion as they are being dried, to a larger or lesser extent, whereby the heating gases become charged with very fine substances which must subsequently be precipitated again by means of expensive dust removing equipment. Besides, the energy consumption of this method is excessive.
Second, there are processes in which the solids, which are not yet dried, are combined with the liquified bituminous binder and the components are then mixed, while intensively heating the components that are being mixed. Such a process is disclosed in German Pat. No. 1,594,815. In using said known process, the binder is fed to the mineral solids, which are not yet dried and usually have a moisture content of 3 to 5% of H.sub.2 O, in a mixer, preferably through atomizing units, in such a manner that there is sufficient random distribution of this binder on the surfaces of the mineral substances.
When the addition has ended and the desired state of mixing is achieved, the mixture is fed, in the case of a batch type operation into a compensating tank and a discharge device. In a continuous operation the mixture is supplied into a dryer, preferably a drum dryer, and whereby the mixing continues in this dryer, while simultaneously heat is supplied intensively to provide the prescribed mixing temperature appropriate for the particular mixture or rather binder.
These mixing plants, which provide a reversal of the sequence of the individual process steps, which may be called the "classical" sequence, with reference to the bituminous compounding first described above, are still employed even today to the satisfaction of the operators, even though the originally expected freedom from dust cannot be called complete, since the point in time at which the binding of the very fine dust to the coarser grain by water ceases is not always exactly the same as that at which subsequent binding by the bituminous binder takes place. Uninterrupted binding of the dust to the coarser particles, regardless whether by water or binder, is, however, a prerequisite if the bituminous mixture compounding is to be free of dust to the maximum extent.
A particularly economical use of this process in the case of a continuous mixing operation was achieved, while accepting a relatively low expenditure for the dust removal in some cases, by the further development of the equipment necessary for executing the process. This development particularly involved omitting a separate mixer and performing the process in a single drum type combined mixing, drying and heating device as is described in particular in German Pat. No. 2,102,328.
Such an operating procedure which provides for a continuous metering of the binder as a function of the result of a continuous weight determination of the mineral substances is, however, not always usable or even permitted in certain countries of central Europe, in spite of the possibility, which exists today, of taking into account the moisture content of the solids by an automatic correction of the control value. This is so because a continuous weighing and measuring cannot give the almost exact accuracy of batchwise dosing. Furthermore, due to the continuous weighing and measuring it is difficult to adapt continuously operating plants to changed recipes, which is frequently necessary in the case of relatively small construction contracts or in the case of stationary plants. Besides, such changes in a continuously operating plant require a prolonged adaptation phase.
Summarizing, it may be said to these prior art developments that they indeed at least reduce the expenditure for the dust removing equipment. On the average, this reduction of expenditures corresponds, in the energy balance sheet, to one KW per ton of mixture produced. However, the visible emission of so-called blue vapors, caused by the direct contact of the burner gases with the binder and comprising a proportion of low-boiling hydrocarbons, from the bitumen, which can scarcely be detected analytically, again reduces the value of this development since maintaining a good appearance is essential. Such appearance requires the removal of the blue smoke at substantial costs.
It is also taken into consideration, in this context, that up to 40% of the present investment costs for a stationary compounding or mixing plant must be used to satisfy environmental protection conditions. Thus, the prior art has not yet been leading in the direction of a satisfactory solution.
In addition to these significant conditions to which prior art road construction is subject, one must take into account the increases is heating oil costs caused by the energy crisis which has meanwhile occurred, and which has caused approximately similar cost increases for the bituminous binder.
In this connection, it is appropriate to define the energy requirement necessary for the classical mixing of one ton of mixture. The following assumptions are made:
plant capacity 120 tons of mixture per hour; PA1 minimum mixing temperature 140.degree. C. and 1,000 kg batches; PA1 additional amount of water (lower limit) of about 30 kg for each 1,000 kg batch; PA1 about 60 kg of bitumen for each batch, one third of the heat content of the bitumen is used to maintain the temperature of the hot-delivered binder; PA1 specific heat of the bitumen: 0.5.times.4.19.times.10.sup.3 Joule; PA1 specific heat of the mineral content: 0.2.times.4.19.times.10.sup.3 Joule; PA1 specific heat of the water: 1.0.times.4.19.times.10.sup.3 Joule; PA1 water vaporization heat: 540.times.4.19.times.10.sup.3 Joule.
Without taking into account the efficiency of the unit, the following heat requirement Q results for a conventional plant under the foregoing assumptions.
______________________________________ 1. Heating the mineral substances, including the filler (940 .times. 0.2 .times. 140) 4.19 .times. 10.sup.3 = 110,280 .times. 10.sup.3 Joule. 2. Maintaining the binder temperature (60 .times. 0.5 .times. 140)/3 .times. 4.19 .times. 10.sup.3 = 5,866 .times. 10.sup.3 Joule. 3. Heating with water (140 .times. 30) 4.19 .times. 10.sup.3 = 17,598 .times. 10.sup.3 Joule. 4. Evaporating the water (540 .times. 30) 4.19 .times. 10.sup.3 = 67,878 .times. 10.sup.3 Joule. Q (conventional) 201,622 .times. 10.sup.3 ______________________________________ Joule.
This requirement corresponds to about 4.8 kg of extra-light heating oil/ton, which means an actual requirement of about 6 kg of heating oil/ton at an efficiency of 80%.
Moreover, the installed power of 310 KW, on the average, for a 120 ton unit, that is to say 310/120=2.58 KW/ton, is to be taken into consideration proportionately in establishing the energy consumption for prior art batch methods.
This heat requirement break-down which has been determined for a low mixing temperature and for a low water content, and the normal requirement of installed power for the compounding plant may increase drastically if the mixing is carried out under rather "wet" conditions. Nevertheless, this evaluation does not appear unrealistic, since the mineral substances, especially in the case of stationary plants, are taken, to an ever increasing extent, from supplies which are roofed over or covered, and are fed to the unit.